A normal requirement within the automobile sector is that the metallic components used for manufacture must be protected against corrosion. The requirements concerning the corrosion prevention to be achieved are very stringent, especially as the manufacturers often give a guarantee against rust perforation over many years. Such corrosion prevention is normally achieved by coating the components, or the substrates used in their manufacture, with at least one coating apt for the purpose.
A disadvantage of the known coating methods, particularly affecting the known methods employed within the automobile industry, is that these methods normally envisage a phosphatizing pretreatment step, in which the substrate for coating, after an optional cleaning step and before a deposition coating step, is treated with a metal phosphate such as zinc phosphate in a phosphatizing step, in order to ensure adequate corrosion prevention. This pretreatment normally entails the implementation of a plurality of method steps in a plurality of different dipping tanks with different heating. During the implementation of such pretreatment, moreover, waste sludges are produced, which burden the environment and have to be disposed of. On environmental and economic grounds, therefore, it is especially desirable to be able to forgo such a pretreatment step, but nevertheless to achieve at least the same corrosion prevention effect as achieved using the known methods.
EP 2 405 035 A1 and JP 2011-057944 A each disclose an electrophoretically depositable coating composition comprising 100 to 1000 ppm and 100 to 5000 ppm, respectively, of trivalent bismuth ions, and an aminopolycarboxylic acid in 0.5 to 10 times and 0.1 to 5 times, respectively, the molar concentration, based in each case on the molar concentration of the trivalent bismuth ions. Within the coating compositions disclosed therein, the trivalent bismuth ions are in dissolved form. A disadvantage of the coating compositions known from EP 2 405 035 A1 and JP 2011-057944 A, however, is the need first, in a separate, upstream method step, to prepare a corresponding water-soluble bismuth-containing compound capable of releasing the trivalent bismuth ions, starting from water-insoluble bismuth precursor compounds such as bismuth oxide and from suitable aminopolycarboxylic acids as complexing agents. This water-soluble bismuth-containing compound thus prepared is only then added to the coating composition. Bismuth salts of this kind in solution in water, which following their preparation are added to a coating composition which is depositable electrophoretically on application of a voltage in a range of 100-400 V, are known from EP 1 000 985 A1, moreover.
Cathodicially depositable bismuth-containing coating compositions which can be deposited onto a suitable substrate in a one-stage coating step are known from, for example, WO 2009/021719 A2, WO 2004/018580 A1, WO 2004/018570 A2, EP 0 642 558 B2, and WO 95/07319 A1: the addition of water-insoluble bismuth subnitrate in electrophoretically depositable coating compositions is known from WO 2009/021719 A2. The bismuth subnitrate used acts here as a crosslinking catalyst. WO 2004/018580 A1 discloses the use of water-insoluble bismuth subsalicylate in coating compositions. The use of bismuth subsalicylate or bismuth ethylhexanoate in coating compositions as a bactericide is described in WO 2004/018570 A2. Water-soluble bismuth compounds are known, moreover, from EP 0 642 558 B2 and WO 95/07319 A1.
A need exists for electrophoretically depositable coating compositions for at least partial coating of electrically conductive substrates with an electrocoat material that permit—especially with a view to forgoing the normally implemented phosphatizing pretreatment step—a more economic and more environmental coating method than conventional coating compositions used, while being nevertheless suitable at least in equal degree for achieving the corrosion prevention effect necessary for such compositions.
It is an object of the present invention, therefore, to provide a coating composition for at least partial coating of an electrically conductive substrate that has advantages over the coating compositions known from the prior art. In particular it is an object of the present invention to provide coating compositions which permit a more economic and/or environmental coating method than conventional coating compositions used. In particular it is an object of the present invention, moreover, to provide a method which allows more economic and/or environmental coating than conventional coating methods, which, in other words, makes it possible, for example, to forgo the phosphatizing which must normally be carried out by means of a metal phosphate even prior to deposition coating, but with which, nevertheless, at least the same, and more particularly an enhanced, corrosion prevention effect can be achieved than is achieved with the normal methods.